The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

Go to Firefox > Help > Troubleshooting Information.

Click the "Reset Firefox" button.

Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.

Follow-up: I've been using the "Save with URL" (0.2.12) Add-On for quite a few years now. It's simple, great, and performs as advertised. Well, apparently 18.0.1 did it in.

A bit of digging suggested the problem might be with an add-on. So I slowly went through them selecting possible candidates, turning them off, restarting, saving a test web page. After turning off SaveWithURL, restarting, and saving the test page. . . voila! FF saved the page as a Web Page Complete. Put it back on, yada yada yada, didn't save the page. I'll let the author of the Add-On know about the problem. I've switched to "Saved From URL". It's less elegant but provides the info I want.